The tournament brings together the top eight players in the world. They are split into two groups and two from each advance to the semifinals.
The Red Group pits Serena Williams against Li Na, Victoria Azarenka and Angelique Kerber, while the White Group has Maria Sharapova competing against Agnieszka Radwanska, Sara Errani and Samantha Stosur. Petra Kvitova withdrew after losing her first round robin match to Radwanska.
Held at Istanbul's Sinan Erdem Dome, the tournament kicked off on Tuesday with a 6-4, 6-1 win by Serena Williams against Angelique Kerber.
In White Group matches, Agnieszka Radwanska defeated titleholder Petra Kvitova 6-3, 6-2, and Maria Sharapova defeated Sara Errani 6-3, 6-2 in a repeat of the French Open final.
On Wednesday Maria Sharapova secured her spot in the semifinals of the WTA Championships in a three-hour match against Agnieszka Radwanska.
Serena Williams extended her dominance over the top-ranked player in the world on Thursday, defeating Victoria Azarenka 6-4, 6-4 at the WTA Championships.
China's Li Na beat Angelique Kerber of Germany 6-4, 6-3 in another Red Group match to retain a slim chance of advancing.
Sara Errani beat Samantha Stosur 6-3, 2-6, 6-0 in the White Group. Li's first win in two round-robin matches means she can still reach the semifinals, depending on other results in her Red Group.
Williams and Maria Sharapova, who didn't play Thursday, have already qualified for the last four.
Sharapova, the French Open champion, could still take the No. 1 ranking away from Azarenka but has to win the tournament undefeated - and hope Azarenka loses against Li on Friday.
Friday's game pit Azarenka against Li, Radwanska against Errani and Sharapova against Stosur.
The semifinals will be played on Saturday, with the finals to determine the ultimate champion to be played on Sunday.